High impedance power supply



Nov. 3, 1953 D. E. SUNSTEIN ET AL 2,658,117

HIGH IMPEDANCE POWER SUPPLY Filed Nov. 16, 1949 JNVENT RS flue/7 a mu5m? .OHl/ID a. wave/0 Paten'ted Nov. 3, 1953 Davidlil. S'unstein,vCynwyd, and 'rallen CrMlinster, -H atboro, -Pa., assignors to PhilcoCorporation,

Philadelphia, 'vania Pa., a corporation of Pennsyl- Application November16, 1949, Serial No. 127,61 4

4 Claims.

invention herein described and claimed is high relative to that of theload impedance.

As is known, high-impedance-power supplies may be employed to advantagewhere it is desired "to maintain the amplitude of the alternatfing loadcurrent substantially constant irrespective of variations in the loadimpedance. Highimpedance power supplies may also be advantageously.employedwhere it is desired that, in response to a varying controlvoltage, the load currentvaries linearly desp' e the fact that the loadmay be inherently non-linear.

The A.-C. impedance of a power supply may, of course, be increased bythe addition of a series resistor ofhigh value. Theinsertion of a seriesresistor has the disadvantage, however, of simultaneously increasing theD.-C. resistanceoi the supply, thus making it necessary to provide ahigher D.-C.. supply voltage than would otherwise be required to deliverthe desired amount of current to the load. i

In accordance with the presentv invention,- a

high-impedance A.-C. power supply is provided which requiresbutrelatively low D.-C. supply voltage to deliver the desired amount ofcurrent to the load.

. .We have knowledge of one prior art circuit capable of functioningassuch a highrimpedance power supply without requiring a high supplyvoltagemsiich a circuit is shown and described in the textbook VacuumTube Amplifiers edited by George Valley, Jr. and Henry Wallman, andpublished in 1948 by McGraw-Hill Book Company,. Inc. as volumel8 of theRadiation Laboratory Series. On pages 433-434 of that book there isdescribed and illustrated (Figure 11.18) a constant-current devicefunctioning as a pentode. load resistor.

The present invention constitutes an improvement over the prior artcircuit shownand described in the above textbook; and to facilitateanunderstanding .of the present invention, a brief description of thatprior art circuit will first be'given.

. In the prior art circuit referred to above, a pen: tode is shownhaving acathode-loaded triode connected between. the source ofdirect-current plate voltage, and the anode of the pentode. Thecathode-loaded triode ,is biased positively by a battery connectedbetween thegrid of the triode: and the pentode-anode end of the cathodeload resistor.

As will be readily understood by those skilled in .the art, thepositivelybiased cathode-loaded triode has a very high impedance toalternating current but a relatively lowimpedanceto direct current-It-will be seen then that, if theoutput loadimpedanceof the pentode beconnectedebetween the pentode-anode and ground, the impedance of thepower supply will-comprise the high impedance of the pentode in parallelwith the high alternating-current impedance of the cathodeloaded triode.Thus,while the A.-Q. impedance of the power supply will be high, a,relatively low value of DL-C. plate voltage will be sufiicient, todeliver a relatively high value of current to the load.

The fact that a relatively'low va1ue,;,of ;.D.-C. plate voltage may ;beemployedis advantageous from a cost standpoint, since; it ispossible touse standard types of low voltage tubes as well as other lessexpensivecircuit elements Utilization; of the prior; art:circuitarrangement has however, been impeded bygthe necessity of usingeither a battery or a, floating power supplyto provide the constantpotential difierence required to obtain between the grid .;of .thecathode-loaded triodeand the pentode-anode end of; the cathode loadresisto r. Neither-of these forms of -constant potentiahdssatisfactoryefor centain applications. For;;example, while a highi p a eo er-supplrm y b6; able for: use in an. airborne radar-installation,tosupply current to the sweep circuits ofa cathode-ray display'tube, theuse of a .battery as asourcez-of constant ,pQtentialmay, be undesirablebecause of aging and weight considerations. A floatingpowensupply'requirestheeuse of either a battery or a high-voltage powertransformer and hence may be likewise objectionable as too bulky andheavy. R v 1 The present invention provides novel electronic means forproviding a constant potential, difference between the grid ofthecathode-loadedtriode and the pentode-anodeend of the cathode loadresistor, neither a battery nor a transformer being required. 7 s 1:

It is theprimary'object-of this invention to provide a novel andimprovedpower supply whose A.-C. impedance is high relative to the loadimpedance. I

A, more specific object of this invention is to provide an improved formof-low-voltage'highimpedance power supply which isless bulky and lessweighty and which requires less maintenance than prior art low-voltagehigh-impedance power supp1ies. "1 These and other objects, advantagesand features of the present invention will become more clearly evidentfrom the following description when considered in connection with theaccompanying single figure of drawing showing a pre-- ferred form ofhigh-impedance power supply embodying the present invention.

Referring now to the drawing, the preferred embodiment of the new andimproved power supply comprises the elements contained within thedashed-line rectangle and includes a pentode II), a triode II, a triodeI2, and a voltage-regulator tube I3. In the preferred embodiment,pentode Ill operates as a triode, its screen grid being connecteddirectly to its anode, and its suppressor grid being connected to itscathode. While these connections reduce the inherently high impedance ofthe pentode, the tube I is reestablished as high-impedance device byvirtue of the cathode load resistor I4. This arrangement is advantageousin that it permits the eil'ective plate impedance of tube I0 to becontrolled, over a wide range, through selection of the value of thecathode load resistor I4. For, as is well understood by those skilled inthe art, the effective plate impedance is related to the cathode loadimpedance in the manner indicated by the equation Zp=T1J+ +1) Re where Z=the effective plate impedance r =the internal plate resistance of thetube Rk=cathode load impedance, and

,l=the amplification factor of the tube.

The anode of tube It is connected to a suitable source of plate-supplyvoltage, B++, by way of the cathode-loaded triode II, resistor Iconstituting the cathode load.

The grid of triode I I is, in accordance with the present invention,maintained at a substantially fixed potential with respect to the anodeof tube ID by novel means which include the voltageregulator tube I3 andthe cathode-follower tube I2.

The voltage-regulator tube I3 may be a conventional two-electrodegas-filled tube having the well-known property of maintaining asubstantially constant voltage across its electrodes despite variationsin the amount of current flowing therethrough. The anode of thevoltageregulator tube I3 is connected directly to the grid of triode H,and is also connected, by way of resistor IE, to the source ofplate-supply voltage, B++. The cathode of the voltage-regulator tube I3is connected to the cathode of the cathodefollower tube I2. The grid ofthe cathode-follower tube I2 is connected directly to the anode of tubeIll. The plate of cathode-follower tube I2 is connected to a source ofplate-supply voltage, B+, whose potential may be lower than that of thepreviously-mentioned source of plate voltage, B++. Resistor IIconstitutes the cathode load of tube I2.

Since tube I2 is operating as a cathode follower, the positive potentialof its cathode follows cophasally any variations which may occur in thepotential of the anode of tube I0. And since the voltage across theregulator tube i3 remains substantially constant, the voltage on thegrid of tube I I will also vary cophasally with the voltage on the anodeof tube ID, the voltage difierence between the grid of tube II and theanode of tube Ill therefore remaining substantially constant. Statedanother way, the potential of the grid of tube II is maintained positivewith respect to that of the anode of tube In by an amount substantiallyequal to the voltage drop across the regulator tube l3, the voltagedifference between the grid and the cathode of tube I2 being negligible.And the potential of the grid of tube I I, relative to that of thecathode of tube II, is preferably such as to permit tube II to conducton the linear part of its grid-voltage-plate current characteristic.

By the arrangement described above, the anode of tube I0 is suppliedwith D.-C. plate voltage through means offering high impedance toalternating current but low impedance to direct current. Thealternating-current impedance of the cathode-loaded tube I I is givenapproximately by the same equation as that given above, namely,

where the values of Zp, r a and R1; are those pertaining to thecathode-loaded triode II.' Thus. by suitable choice of Br and ,lL, theA.-C. impedance of the cathode-loaded triode II may be made as large asdesired.

It will be seen, then, that if the useful output load I9 be connectedbetween the anode of pentode I0 and ground, the internal impedance ofthe power supply comprises the high A.-C. impedance of tube ID inparallel with the high A.-C. impedance of the arrangement comprised oftubes II, I2 and I3 and their associated elements.

The function of the cathode-follower tube I2 will become clear byconsidering what the effect would be if the voltage regulator tube I3were to be connected directly between the grid of tube .II and the anodeof tube II]. If that were done,

the resistor I6 and the regulator tube I3 would be serially connected inshunt with the cathodeloaded tube II, and since the A.-C. impedance ofthe resistor It and regulator tube I3 is relatively low, the impedanceof the power supply would be low rather than high. In the arrangementproposed by the present invention, the high A.C. input impedance of thecathode-follower tube I2 is effectively in series with the relativelylow A.-C. impedance of the regulator tube I3 and resistor I6, thusproviding a high A.-C. impedance in shunt with the cathode-loaded tubeII.

The provision of the cathode-follower tube I2 creates no additionalplate-voltage supply problem, since the plate potential, B+, requiredfor the cathode-follower tube I2 may be obtained from the source ofplate voltage, B++, by way of a suitable voltage divider.

It will be understood that, in a practical arrangement of this type,provision must be made for exciting, in the power supply, thealternating current signals which it is desired to deliver to the load.

This is preferably accomplished by providing a source of alternatingcurrent control signals, which may here be schematically represented bybox I8 and which is usually coupled to the control grid of tube It]. Thecurrent through load I9 will then vary linearly in response to thevariations of the aforesaid control signals and its amplitude will besubstantially independent of variations in the load impedance.

Alternatively, the A.-C. control signals may, of course, be generatedwithin the power supply itself by incorporating tube ID in a suitableregenerative circuit.

It will be understood that the choice of tube characteristics, platevoltage, and other circuit parameters will depend upon the specificvalues of current output, or degree of linearity, which it is desired toobtain. Determination of these parameters is well within the skill ofthose versed in the art.

By way of example, in a particular case it was desired to supply a totalcurrrent swing of 2 milliamperes to a slightly non-linear load inresponse to a 200 volt swing in control voltage. In that case, weemployed a triode-connected 6AG5 pentode for tube I0, the two halves ofa 2051 double triode for tubes II and I2, and a 5651 voltage-regulatortube for tube 13. The platesupply voltages B++ and 3+ were respectively600 volts and 450 volts positive with respect to ground. Resistor I4 hada value of 100,000 ohms; resistor l5, 70,000 ohms; resistor I6, 86,000ohms; and resistor 40,000 ohms.

While apparatus constructed according to our invention has beendescribed with reference to a single embodiment it will be understoodthat alternative arrangements will suggest themselves to those skilledin the art. We, therefore, desire our inventive concept to be limitedonly by the scope of the appended claims.

We claim:

1. A high-impedance alternative-current power supply comprising a firstvacuum tube having at least anode, cathode and control grid electrodes;means for applying a unidirectional plate-supply voltage to the anode ofsaid first tube, said means including a second vacuum tube having atleast anode, cathode and control grid electrodes, said cathode of saidsecond tube being connected to said anode of said first tube by way of aresistor; means for maintaining said control grid of said second tube ata predetermined potential with respect to the anode of said first tube,said lastnamed means comprising a voltage-regulator tube having anodeand cathode electrodes and a cathode-follower circuit, said circuitincluding a tube having at least anode, cathode and control gridelectrodes, the anode of said regulator tube being connected to thecontrol grid of said second tube, and to the anode of said second tubeby way of a resistor, the cathode of said regulator tube being connectedto the cathode of said tube in said cathode-follower circuit, and thecontrol grid of said cathode-follower tube being connected to the anodeof said first tube; means for applying an alternating-current signal tothe control grid of said first tube; and means for taking an output loadcurrent from the anode of said first tube.

2. Apparatus according to claim 1 characterized in that said first tubehas pentode electrodes, its screen grid being connected to said anodeand its suppressor grid being connected to said cathode.

3. In a high impedance alternating-current supply, a first vacuum tubehaving at least anode, cathode, and control grid electrodes, a secondvacuum tube having at least anode, cathode and control grid electrodes,the cathode of said first tube being connected to the anode of saidsecond tube via a resistor; and a cathode follower circuit comprising athird vacuum tube having at least anode, cathode and control gridelectrodes and a cathode load resistor connected to said lastnamedcathode, said last-named grid constituting the input electrode of saidcathode follower circuit and being connected to the anode of said secondtube and said cathode of said third tube constituting the outputelectrode of said cathode follower circuit and being connected to thegrid of said first tube, thereby to reproduce variations in the anodepotential of said second tube cophasally at said last-named grid.

4. Apparatus according to claim 3 and further including means formaintaining a constant potential difference between said output elec-'trode and the grid of said first tube.

DAVID E. SUNSTEIN. ALLEN C. MUNSTER.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,206,123 Rinia July 2, 1940 2,210,393 Braden Aug. 6, 19402,326,614 Bowman Aug. 10, 1943 2,398,916 Brewer Apr. 23, 1946 2,434,939Levy Jan. 27, 1948 2,456,638 Kenyon Dec. 21, 1948 2,519,377 Jenkins Aug.22, 1950

